BREAKING: Refactor retry argument attributes

[jmgate]

Mypy doesn't like that the retry argument attributes are defined dynamically when a class is instantiated, so it'll complain that they don't exist when a subclass developer is trying to modify them when overriding/extending the parser. If instead of having separate attributes per registered stage, we just had a dict where the keys are the stage names, mypy would be happy.

Note: This will be a breaking change.